Laser-based wheel alignment system

ABSTRACT

A device and method to align the wheels of motor vehicles. A beam of collimated light from a low power visible laser is split into two generally parallel beams by a partially transmitting mirror and a series of plane front surface mirrors positioned in front and along side the vehicle being serviced. The two beams are reflected from a set of plane mirrors attached to the rims of the wheels to be aligned. The vertical and horizontal angles of each of the wheel-mounted mirrors are set by calibrated lead-screws to cancel the angular displacement of the wheels expected when the wheels are properly aligned. The reflected beams are imaged through a large aperture beam combiner on a common viewing screen. Correct toe-in and camber settings are indicated when the laser beams reflected from the wheel-mounted mirrors overlap at the center of the viewing screen. The caster settings of the front wheels are measured by observing the angle at which the reflected spots travel across the screen when the steering wheel is turned the system can be used to align the front wheels only, or alternatively all four wheels simultaneously.

BACKGROUND OF THE INVENTION

The present invention relates generally to wheel alignment systems, andmore particularly to laser-based wheel alignment systems.

The axes about which the front wheels of an automobile or truck turn asit travels down the road must be carefully set to minimize tire wear andinsure safe and stable handling characteristics. The orientation ofthese axes is determined by three angles: (1) the toe-in angle, whichspecifies the angle between the rim of the wheels and a line drawnparallel to the direction in which the car is pointed; (2) the camberangle, which specifies the angle between the rim of the wheels and thevertical; and (3) the caster angle, which specifies the angle betweenthe vertical and the axis about which the individual wheels turn whenchanging direction. These angles are typically specified individuallyfor each wheel and for each model and make of vehicle, and must beperiodically tested and reset as the vehicle and tires age to insurecontinued economic and safe vehicle performance.

The measurement of these angles has represented a growing challenge tothe shops and service stations responsible for alignment service. Whilesimple side-slip gauges were successfully used in the past to check andset toe-in, the readings obtained with such gauges depend on the type oftire installed on the vehicle. In particular, side-slip gauges appear togive widely diverging and erroneous readings for radial tires.

Further, while presently available mechanical alignment machines arecapable of measuring the toe-in, camber, and caster angles, the specialramps, gauges, and electronic hardware and software required foroperation are beyond the means of many service stations who wish toperform this work.

SUMMARY OF THE INVENTION

The present invention provides a simplified, compact, accurate, andeconomical alignment system capable of measuring the toe-in, camber, andcaster angles of the wheels of road-going motor vehicles. The inventionuses a simple optical system, which substantially reduces both the costof the equipment and the space required to perform wheel alignmentservice.

According to a broad aspect of the invention, a single, highlycollimated, beam of light from a low power visible laser is split intoseveral parts which are then directed by a sequence of plane (preferablyfront surface) mirrors to define a series of reference beams in a planeparallel to the surface on which the vehicle being serviced is parked.The plane mirrors include fixed mirrors for directing the beams to thefront wheels and moveable mirrors for directing the beams to the rearwheels (if required).

These reference beams are set up to strike the surfaces of a set ofplanar retroreflectors mounted on the wheels being serviced. Theorientation of the retroreflectors is set so that, when the wheels areproperly aligned, the reflected laser beams will exactly retrace thepaths of the initial reference beams. The reflected beams are thencombined using a large aperture partially transmitting mirror and thepositions of the reflected beams are observed visually on an opaque orground glass screen.

The invention utilizes a beamsplitter (such as a multilayer dielectricor pellicle-type beamsplitter) to divide the laser beam into twogenerally parallel beams which are directed towards, and reflected fromthe wheel-mounted retroreflectors. The orientation of each of theretroreflectors is set by the service technician to cancel the verticaland horizontal angular offsets expected when the toe-in and camberangles are properly set. The beams returned from the retroreflectors arecombined either in the same or a second, independent beamsplitter toproduce a visual image of the configuration of the reflected beams on aviewing screen. A large aperture beam combiner is used to permit theacquisition of beams reflected from badly misaligned wheels and topermit service to be performed on vehicles which are parked skew to thecenterline of the system.

The position of the reflected laser beams on the screen relative to thecenter of the screen and/or to each other indicates the deviation fromcorrect alignment. Perfect alignment is obtained when the laser beamsoverlap on the viewing screen to form a single spot which is verticallycentered on the viewing screen. While an angular offset of the vehiclewill cause the merged spots to move horizontally across the screen, thismotion will not cause the merged spots to separate. Thus wheel alignmentcan be measured even when the vehicle is not perfectly aligned with theaxis of the alignment system.

The plane mirrors used to guide the incident and reflected laser beamsto and from the wheel-mounted retroreflectors define a folded opticaltransport system for the incident and reflected laser beams which eitherpreserves or reverses the left/right orientation of the incident andreflected beams at the viewing screen, depending on whether the numberof reflecting surfaces is even or odd. The number of mirrors in the leftand right branches of the system must both be either even or odd so thatthe beams reflected from the left and right and front and rear wheels ofa vehicle can be compared on the viewing screen.

The path lengths for the beams reflected from the left and right frontwheel-mounted mirrors to the viewing screen must be substantially equalso that the spots on the viewing screen remain overlapped if the axis ofthe vehicle is displaced in angle from the centerline of the alignmentsystem. It is also necessary to employ one or more lenses along theoptical path rear wheel-mounted mirrors to the viewing screens so thatthe spots created on the viewing screens by the beams reflected fromfront and rear wheels remain overlapped when the axis of the vehicle isdisplaced in angle.

The orientation of the mirrors must be set so that the incident beamsfrom the laser and beamsplitter to the wheel-mounted mirrors each lie asnearly as possible in a single plane parallel to the surface of thefloor on which the vehicle rests, and are preferably each eitherparallel or normal to the initial laser beam.

The use of a series of beamsplitters and mirrors to create a system oflaser reference beams, the use of tiltable wheel-mounted retroreflectorsto null the angular deflection of the reflected beams caused by thedesired values of toe-in and camber, and the use of a beam combiner tocompare the angular orientation of the beams reflected from the wheelsbeing serviced provides a simple and accurate way to align the wheels ofthe vehicle being serviced, which eliminates the requirement in prioralignment systems to individually measure the alignment of each wheel.Further, this system is substantially unaffected by errors in theplacement or orientation of the vehicle being serviced.

In the preferred embodiment of the invention, the same beamsplitter isused to divide and recombine the laser beams. A ground glass screen isused to directly view the reflected and combined beams, while a closedcircuit TV camera is used to transmit the image of the combined beams toone or more remote locations for viewing by service and supervisorialpersonnel or by the customer.

The readout obtained from this simplified system is particularlystraightforward, and readily adapted to use by personnel working underthe car or in other inaccessible locations, or via closed circuittelevision at a distance from the vehicle. In addition, the readout canbe readily understood by untrained personnel, including customers whowish to observe the alignment procedure or to verify the initial orfinal states of alignment of their vehicles. The opportunity forcustomers to observe the alignment process is a significant salesfeature, particularly for customers with a personal interest in thecondition or performance of their vehicles.

A further aspect of the invention relates to a preferred mounting forthe retroreflectors to the rims of the wheels to be aligned. A devicefor mounting the mirror to the wheel comprises a flat reference platehaving at least three pins for registering the plate to the wheel rimparallel to the wheel plane. A plurality of permanent magnet assembliesengage the wheel's studs or lug nuts to hold the reference plate inplace. The retroreflector is carried by a kinematic mount rotatablymounted to the reference plate. The kinematic mount can be levelled, andits angles set so that the retroreflector is vertical and parallel tothe retroreflector on the opposite wheel when the wheel angles arecorrectly adjusted.

A further aspect of the invention relates to the magnet assembly thatengages the stud. The assembly includes a permanent magnet magnetizedaxially relative to the lug nut. A cylindrical shroud of highpermeability material surrounds the permanent magnet and extends beyondit to surround the stud. This provides a return path for the flux andmaximizes the attractive force between the magnet assembly and the stud.

A further aspect of the invention relates to a preferred mirror mountfor setting the position and orientation of the plane mirrors oppositethe wheels of the vehicle under test. While the orientation of the fixedmirrors is set by conventional surveying techniques, the inventionprovides a simple and effective way to set the moveable mirrors in thesystem at 45° to the nominal axis. This facilitates the set-up of thealignment system, particularly when checking the alignment ororientation of the rear wheels of a vehicle.

The mirror is mounted to a platform that rotates relative to anintermediate element such as a ring. The intermediate element itselfrotates relative to a plate. The platform and intermediate element areformed with detents for establishing desired relative orientations, forexample 0° and ±45°. The intermediate element can be locked at anyposition relative to the plate. In use, the platform is rotated to the0° detent, and the intermediate element is rotated to set the mirrorperpendicular to the beam axis. The intermediate element is locked inposition, and the platform is then rotated to engage the detent at +45°or -45° as required.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and to the attached drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top schematic view of a laser-based wheel alignment systemaccording to the present invention;

FIGS. 2A and 2B are top and side views of the laser, beamsplitter andviewing screen assembly;

FIGS. 3A and 3B are top and side views of the mirror and lens mount;

FIGS. 4A and 4B are front and side views of the device for mounting theretroreflectors to the wheels;

FIGS. 5A and 5B are exploded isometric and cross-sectional views of thekinematic mount for holding and aligning the retroreflector;

FIGS. 6A and 6B are exploded isometric and cross-sectional views of themagnet assembly for holding the retroreflector mounting device againstthe wheels;

FIGS. 7A, 7B, 7C, and 7D are perspective and top views and twocross-sectional views of a preferred mounting for the moveable 45°deflection mirrors; and

FIG. 8 shows the paths traced by the beam spots on the screen as thesteering wheel is turned.

DESCRIPTION OF SPECIFIC EMBODIMENTS Optical System Overview

FIG. 1 is a top schematic view of a laser-based alignment system 10 foraligning the wheels of a vehicle. The system is characterized by an axis11 along which the vehicle is placed as closely as is practical. Thevehicle itself is not shown, but its left and right front wheels 12L and12R, and its left and right rear wheels 13L and 13R are. Although notshown, access to the suspension components of the vehicle being servicedcan be provided by means of a pit under the vehicle in the slab on whichthe vehicle is parked. To secure access to the suspension system of thevehicle, the pit should extend either partly or completely along thelength of the vehicle. The design of such pits is well established inthe automotive service industry, and will not be described further.

Each wheel carries a retroreflector 15 that is mounted so as to beperfectly vertical when the toe-in and camber angles are correctly set.A laser 20 produces a collimated beam of visible light which is splitinto two equal-intensity beams by a beamsplitter 22. One of the beams,designated 25 is reflected by the beamsplitter while the other beam,designated 27 passes through it. Beam 25 encounters plane mirrors 30 and31 which direct it rearwardly along the left side of the vehicle. Planemirrors 32 and 33 direct a portion of beam 25 to the left wheels of thevehicle. Similarly, a plane mirror 41 directs beam 27 along the rightside of the vehicle. Plane mirrors 42 and 43 direct the laser beams tothe right wheels. Mirrors 33 and 43 are optional, being required only ifthe rear wheels of the vehicle are to be aligned. If such is the case,mirrors 32 and 42 are partially transmitting.

A first pair of lenses 45 are disposed in the beam paths from the rearretroreflectors to account for the possibility that the vehicle is notperfectly positioned parallel to axis 11. A second pair of lenses 46 areplaced in the beam paths from the front retroreflectors. These lensesare optional, and are used to increase the range of angles that can beimaged as will be described below.

As noted above, the two beams are directed along respective paths oneither side of the vehicle by mirrors 31 and 41, which are oriented todirect the beams along generally parallel paths lying in a planeparallel to the surface on which the vehicle rests. All or part of thebeams are then directed to the front retroreflectors by mirrors 32 and42. Mirrors 32 and 42 are set at 45° so that the paths to theretroreflectors are parallel. If both the front and rear wheels of thevehicle need to be aligned, mirrors 32 and 42 must be partiallytransmitting to permit passage of a portion of the beams to pass throughto mirrors 33 and 43 and to the rear wheel-mounted retroreflectors.Mirrors 33 and 43 are set at 45° so that the paths to the rearretroreflectors are parallel.

The laser beams reflected from the wheel-mounted retroreflectors thenpropagate along paths, shown in phantom, to beamsplitter 22 (which nowacts as a beam combiner) and then to a screen 47 and an optional secondscreen to be described below. The laser/beamsplitter assembly ispositioned between mirrors 31 and 41 to equalize the path lengths fromthe left and right retroreflectors to the viewing screen 47.

If the vehicle were positioned exactly parallel to axis 11 and thewheels were properly aligned, the retroreflectors would each be verticalwith their planes parallel to axis 11, and the return beams wouldretrace the forward paths and converge to form a single spot at thecenter of screen 47. Alignment and positioning errors will force thereturn beams to follow the deviated paths shown in phantom leading tothe appearance of multiple spaced spots on the screen which may beoffset from the center of the screen. Accordingly, the beam-combiningbeamsplitter and the viewing screen must have a height and widthsufficient to image the beams reflected from the retroreflectors overthe anticipated range of initial alignment and vehicle positioningerrors. Beam combiners as large as 6" by 12" can be fabricated usingdielectric multilayer films deposited on high quality ground or castplate glass substrates.

The laser source required for operation can operate at any wavelength inthe visible region of the spectrum, so long as its power output fallswithin the limits for safe operation prescribed by the Bureau ofRadiological Safety and OSHA. Typically, a 2 milliwatt helium-neon laserwould be used as this source. Since the division ratio of thebeamsplitters used to divide and recombine the beams produced by thelaser and reflected from the wheels being serviced will vary dependingon the polarization of these beams, to attain a fixed division ratio itis necessary to use a laser with fixed linear polarization orientedeither normal to or in the plane of the beamsplitter.

The wheel-mounted retroreflectors should have the largest possiblediameter consistent with the limits imposed by the mounting structure soas to maximize the range of wheel diameters which can be serviced at agiven height of the laser above the slab on which the vehicle is parked.A diameter of 3 inches or more is recommended since it can be used toservice vehicles with tire diameters varying over a range of 6 inches.

FIGS. 2A and 2B are top and front views of the laser/beamsplitterassembly. Also shown is the second screen 60, mentioned above but notshown in FIG. 1. Laser 20, beamsplitter 22, mirror 30, and screens 47and 60 are mounted on a rigid plate 62. Plate 62 is in turn supported bya wall bracket 63 or the like which is bolted to a rigid masonry wall orpier. Plate 62 is levelled by means of the three kinematic adjustmentscrews 65. The height of the bracket and hence the laser is set by meansof slotted mounting holes 67 and adjustment screws 68 insure that thelaser reference beams strike the reflecting surfaces of the wheelmounted retroreflectors.

Viewing screen 47 may be made of ground glass or plastic. The combined,reflected laser beams are stopped on the ground surface of the screen,and viewed through the screen by the operator. This screen is bestmounted on the side of beamsplitter 22 opposite to laser 20. Screen 60has a non-specular surface and can be installed between the beamsplitterand the laser, provided that a small hole 70 is drilled through thescreen to allow passage of the beam from the laser. This second screenmust typically be inclined at an angle to allow viewing of the screenfrom above or from the side. A closed circuit TV camera 71 transmits theimage of the beam spots on screen 60 to a remote TV monitor 72 for theuse of the service technician or others.

As noted above, lenses 45 are provided to insure that the displacementof the laser beams reflected from the rear wheels of the vehicle as seenon the viewing screen(s) match the displacement of the beams reflectedfrom the front wheels of the vehicle in the event the axis of thevehicle is not parallel to axis 11.

FIGS. 3A and 3B are top and side views showing the mounting for one oflenses 45 and its neighboring mirror (for example, mirror 42). The lensis shown in phantom since it is mounted in a ring 75. The lens andmirror are mounted to a plate 77 which may be leveled and supported inthe same manner as plate 62 in the laser/beamsplitter assembly. Lenses45 must be installed so that their optical axes are centered on the beampaths from the laser to prevent the deflection of the laser beams fromthe laser propagating along these paths.

Lenses 46 are optionally placed in the paths in order to increase therange of angles over which the returning beams can be imaged throughbeamsplitter 22 on the screens. Lenses 46 must also be carefullyinstalled so that their optical axes are centered on the paths toprevent the deflection of the laser beams from the laser propagatingalong these paths.

Lenses 45 and 46 can either be conventional refractive or fresnellenses, or holographic phase gratings fabricated to provide the requiredfocal length at the operating wavelength of the laser source. The lensesshould either be anti-reflection coated or fabricated from suitablelow-index optical materials to minimize reflection losses.

Lens Parameters

The focal length of the lenses 45 is determined by the optical pathlength from the front retroreflectors to the viewing screens, from therear retroreflectors to the screens, and from the rear retroreflectorsto the lenses according to the equation:

    f=L.sub.3 ·(L.sub.2 -L.sub.3)/(L.sub.2 -L.sub.1)

where

L₁ =optical path length from front retroreflectors to viewing screen(s)

L₂ =optical path length from rear, retroreflectors to viewing screen(s),

L₃ =optical path length from rear retroreflector to lens 45.

It is assumed in this equation that the distance from the lenses toscreens 47 and 60 is the same on the left and right hand sides of thevehicle being serviced. Although the focal length computed for lenses 45will vary depending on the wheel base of the vehicle being serviced, itis possible for most passenger cars and light trucks to use a singlelens computed on the basis of the average wheel base to be serviced. Thelenses the need only be changed when servicing vehicles withexceptionally long or short wheelbases. It is, of course, not necessaryto use such lenses at all if only the front wheels of the vehicle needto be aligned.

As an optional feature an additional pair of lenses 46 are placed in theoptical paths between the front retroreflectors and the viewing screensto increase the effective aperture of the viewing screens. If theselenses are not used, reflected beams which deviate from the incidentbeams by an angle θ greater than arctan(A/L), where A is the viewingscreen half width or height and L is the path length from retroreflectorto viewing screen, will miss the viewing screen. If the wheels of thevehicle being serviced are grossly out of alignment, it may beimpossible to image the reflected beams on the viewing screen, andhence, impossible to tell in what direction the toe-in and camberadjustments need to be moved to align the wheels being serviced.

The range of initial alignment errors which can be accommodated by theinvention can be increased by adding lenses 46 between the frontretroreflectors and the viewing screens. The focal length of theselenses is determined by their placement and by the magnitude of thedesired increase in the angular aperture of the viewing screen. Assumingthat it is desired to increase the aperture by a factor M, the focallength of lenses 59 can be computed from the equation:

    f=(M/(M-1))·(1-(L.sub.4 /L.sub.1))·L.sub.4

where

L₄ =distance from front retroreflector to lens 46.

The focal length of lenses 45 must also be changed in this version ofthe invention to preserve the functionality of lenses 45. The focallength of lenses 45 required when lenses 46 are used can be computedfrom the equation: ##EQU1##

Wheel-Mounted Mirrors

FIGS. 4A and 4B are front and side views of a device 80 for mounting oneof retroreflectors 15 to one of the wheels. Device 80 comprises a flatreference plate 81 which is registered to the rim of the wheel by threepins 82 extending perpendicular to the plate from three triangularlylocated mounting points. Portions of the wheel rim are shown in phantomin FIG. 4B. The plate is held against the wheel by a set of magnetassemblies 85 which engage the wheel's studs or lug nuts. The magnetassemblies are mounted on threaded rods 86 with compression springs 87and jam nuts 88, which are set by the operator to adjust the forcegenerated by the magnets and to compensate for the varying heights ofthe studs encountered on differing vehicles.

Plane retroreflector 15 is mounted in a kinematic mount 90 having abubble level 92. Kinematic mount 90 is mounted for rotation about aspindle 95 that extends perpendicular to the plate. The vertical andhorizontal axes of the mount are established by rotating the mount onspindle 95 so as to center the bubble in level 92. The kinematic mountincludes leadscrews 97 and 98, each with a knurled knob, to provide foradjustment of the vertical and horizontal angles of the mirror relativeto the plate. The knobs actuating the leadscrews are calibrated indegrees to permit the operator to set the vertical and horizontal anglesof the mirrors to match the angles listed in the manufacturer'sspecifications for the camber and toe-in of each wheel.

Reference plate 81 is preferably formed with four slots, a pair oflongitudinally opposed slots 100 and 102 and a pair of slots 105 and 107disposed at ±36° from slot 102. While only three slots are required toaccommodate the pins, the four slots are required to allow for thevarying numbers of wheel studs encountered on differing vehicles. Theconfiguration of slots 100, 102, 105, and 207 shown in FIG. 4A is neededto accommodate the 4-, 5-, and 6-stud wheels most commonly encounteredon passenger vehicles and light trucks. In this arrangement, pins 82should be located permanently (but slidably) in slots at 100, 105, and107 while the magnet assemblies can be moved from slot to slot asrequired to accommodate the changes in the positions of the mountingstuds encountered on different vehicles.

FIGS. 5A and 5B are exploded isometric and cross-sectional views ofkinematic mount 80 and retroreflector 15. The basic elements of thekinematic suspension system for the mount include a back plate 120 and afront plate 122, tension springs 125, lead screws 97 and 98, and a setscrew 127. As in prior kinematic mounts, the end of set screw 127 ismachined to the form of a hemisphere which is seated in a hemisphericalcup 128 cut into rear plate 120. Rotation of the set screw in thethreads cut into front plate 122 determines the nominal separation ofthe assembled front and back plates. The end of one of the lead screwsis also cut into the form of a hemisphere which is seated in acylindrical groove 129 cut into the back plate. It is important thatthis groove be cut along a straight line which intersects the center ofthe cup locating the set screw, and that the two lead screws be locatedat 90° relative to each other and the set screw. The end of the secondlead screw can be cut to form a spherical surface of arbitrary radius.The end of this lead screw slides freely along the planar front surfaceof the back plate. Rotation of the two lead screws in the threads cutinto the front plate uniquely and stably determines the vertical andhorizontal angles of the front plate relative to the front and rearsurfaces of the back plate. The front and back plates are held injuxtaposition by means of the tension springs 125 whose free endsproject through clearance holes cut into the front and rear plates. Theends of the springs are fixed by means of pins which pass through theends of the springs and are set into recesses cut into the rear surfaceof the back plate and the front surface of the front plate.

Spindle 95, which is used to set the angular orientation of thekinematic mount is threaded into the rear surface of the mount at oneend, and held at the other end against the rear surface of referenceplate 81 by means of a flat washer 130 which slides without interferencealong the cylindrical surface of the spindle, a belville-type springwasher 132, and a jam nut 133. The length of the spindle, the thicknessof the flat washer, and the spring constant of the belville washer arechosen to hold the rear surface of the back plate of the kinematic mountagainst the front surface of reference plate 81 while permitting thefree rotation of the kinematic mount about the axis of the spindle. Thesliding surfaces of reference plate 81, back plate 120, and spindle 95must be machined to close tolerances, polished and lubricated, orotherwise treated to reduce friction and wear during operation.

Retroreflector 15 is clamped in a conformal cup machined in the frontsurface of front plate 122 of the suspension system by means of a clampring 135 and machine screws 137. In this arrangement, the angularorientation of the front surface of the mirror and the rear surface ofthe rear suspension plate is set by the planarity of the front and rearsurfaces of the mirror and the suspension plates and by the gap betweenthe plates as determined by set screw 127 and vertical and horizontallead screws 97 and 98. It is straightforward to hold the planarity ofthe front and rear surfaces of the mirror and suspension plates tobetter that a minute of arc. Under these circumstances, the angularorientation of the front surface of the mirror is for practical purposesdetermined entirely by the gap between the front and rear suspensionplates, which can be checked for purposes of calibration by means ofstandard gauge blocks, feeler gauges, or micrometer calipers.

The material chosen for retroreflector 15 must satisfy a number ofstringent requirements including dimensional stability, highreflectivity, and resistance to abrasion and impact damage. Thepreferred material for this mirror is chrome carbide, a hard and stablemetallic alloy which is available commercially for use in precisionmetrology laboratories in the form of large diameter, optically polishedcylindrical blanks.

The readout of the angular position of the retroreflector in this mountis secured by observing the angular positions of the knobs for thevertical and horizontal lead screws. As shown in FIGS. 4A and 5A, theknobs are scribed to permit the visual observation of the angle throughwhich the knobs have been turned, and hence, of the change in gapbetween the front and rear suspension plates. The actual calibration ofthe knobs is determined by the pitch of the lead screws and their linearseparation, which quantities must be chosen to match the range in anglesrequired for operation. The thread and spacing of the lead screws andthe pattern for the scribe marks should be chosen to permit the anglesof the mirror to be set to within ±0.05 degrees or better. The sense ofrotation of the lead screws, and hence the signs of the horizontal andvertical angular displacements of the mirror, is determined byobservation of the relative heights of the top surface of the knobs forthe lead screws and the elevated bosses on the front of clamp ring 135.The bosses also serve to protect the knobs from impact damage.

The angular orientation of the kinematic mount about spindle 95 ismeasured by means of bubble level 92 which is cemented into a machinedrecess on the front surface of clamp ring 135. In use, the kinematicmount is rotated as a whole about the axis of spindle 95 until thebubble in this level is centered.

Wheel-Retaining Magnet Assemblies

FIGS. 6A and 6B are exploded isometric and cross-sectional views of oneof magnet assemblies 85. The magnet assembly functions to produce asystem in which the energy stored in the magnetic field varies rapidlywith the separation of the assembly and the stud to which it is applied,thereby maximizing the attractive force between the assembly and thestud.

The magnetic field in the assembly is generated by a slug 140 ofpermanently magnetized material such as samarium cobalt or neodymiumiron. The slug is magnetized along the symmetry axis of the magnetassembly. It is important to choose a material for this applicationwhich is characterized by a high coercive force and a large energyproduct. In the event that a brittle material such as samarium cobalt ischosen, the slug must be encased in a shock-resistant capsule. Thecapsule includes a non-magnetic cylindrical shell 142 and end caps 145and 147. End caps 145 and 147 adjoining the north and south poles of theslug must be fabricated from low carbon steel or an equivalent highlypermeable material to reduce the reluctance of the magnetic circuit andthe volume in which magnetic energy can be stored in the system.

A cylindrical, low carbon steel shroud 150 in the assembly guides thefield from the back of the slug to the outer surface of the lug nutwhich is threaded on to the stud to fasten the wheel to the stud. Themagnetic energy stored in the system is then essentially limited to thespace between the end of the stud and the front pole of the slug, andthe volume of the slug. Neglecting saturation and the air gap betweenthe inside of the shroud and the surface of the lug nut, the storedenergy would fall to zero when the front pole of the slug contacts theend of the stud. The stored energy increases rapidly when the stud andpole are separated due to the increase in volume available for energystorage in the gap between the stud and the pole, and the growth of theH-component of the magnetic field within the slug.

Since the force between the magnet assembly and the stud is determinedby the rate of change of stored energy with the distance of separation,the use of shroud 150 in the assembly substantially increases the forcewhich can be obtained from a given volume of permanently magnetizedmaterial. This both reduces the cost of the magnetic material requiredfor the system and makes it possible to achieve the large attachmentforces required to reliably secure the reference plate, mirror, andkinematic mount to the rim of the wheel.

In addition to its role in increasing the attractive force between themagnet assembly and the stud, the shroud also serves the useful purposeof reducing the external magnetic field of the assembly, and hence thegenerally undesirable attractive force between the magnetic assembly andother tools or steel objects which might accidentally be present in thevicinity of the assembly.

45° Self-Indexing Mirror

FIGS. 7A, 7B, 7C, and 7D are perspective and top views and twocross-sectional views of a mirror assembly 160 for rapidly establishingthe proper 45° angles for mirrors 33 and 43. The mirror assemblyincludes a plate 162 whose inclination can be adjusted relative to asupport plate 163 mounted rigidly on top of a tripod 165. The angularorientation of adjustable plate 162 is set to match the true vertical bymeans of lead screws 170 and 172 and a bubble level 175. The design andassembly details of the kinematic support system whereby plate 162 maybe levelled are conventional and generally follow the principlesdescribed above in connection with levelling plate 62 for thelaser/beamsplitter assembly and for orienting retroreflector 15 relativeto reference plate 81.

Mirror 33 is held by a pair of posts 180 which are rigidly mounted to aplatform 182. Platform 182 is supported on a two-stage rotating assemblyfor rotation relative to plate 162. More particularly, platform 181 ismounted for rotation relative to a ring 185, which is itself mounted forrotation relative to adjustable plate 162. Relative rotation about acommon axis is established by thrust bearings 187 and 190. Posts 180 areset during fabrication to ensure that the front surface of mirror 33 isparallel to the axis of rotation of thrust bearings 187 and 190. A beltdrive assembly including a belt 195, a pulley 197, and a knob 198provides for rotation of ring 185 relative to plate 162.

The angular orientation of mirror 33 in the horizontal plane is set byring 185 and platform 182. Platform 182 is indexed relative to ring 185by a spring loaded ball bearing 192 which may seat in any one of threeconformal hemispherical cups cut into the underside of platform 182 atrelative angles of 0°, +45°, and -45°. In operation, platform 182 isturned relative to ring 185 until ball bearing 192 seats in the cup at0°, and ring 185 is turned by means of the belt drive assembly toreflect the incident laser beam back upon itself. Once that alignment isestablished, the ring is locked in position relative to adjustable plate162 by means of a friction lock 200, and the platform is turned in thedirection required to engage the appropriate 45° detent. In practice,machining tolerances for the indexing system can be held to well belowone minute of arc, a value which is generally adequate for the purposeof the wheel alignment system.

Operating Procedure

To use this device, the vehicle being serviced must be driven onto thefloor slab with the front wheels between mirrors 32 and 42 to a positionat which the laser beams 29 and 30 strike retroreflectors 15L and 15Rmounted on the front wheels of the vehicle. The vehicle must further beparked so that it is approximately centered between the mirrors 32 and42, and so that the longitudinal axis of the vehicle is approximatelyparallel to axis 11, which is defined by the laser beams. Reasonablecare should be taken to park the vehicle close to this ideal positionparallel to the longitudinal axis of the alignment system. To insurethat the vehicle is accurately positioned, reference marks or linesshould be painted or engraved on the floor to guide the vehicle into thearea in which it must be parked.

If only front wheel alignment is to be performed, the vertical andhorizontal axes of the retroreflectors mounted on the front wheels areset and adjusted using leadscrews 48 and 49 as described above, and thesteering wheel turned until the image of the reflected spots on theviewing screen appear near the center of the screen. The camber andtoe-in angles are then adjusted for each wheel until the two reflectedbeams merge at the center of the screen when the steering wheel iscentered.

Once the toe-in and camber angles have been set, the caster angles foreach wheel can be set. FIG. 8 shows the effect of caster on the tracksgenerated on the viewing screens by the spots reflected from the leftand right front retroreflectors when the steering wheel is turned fromits counterclockwise to the clockwise limits. While the spots merge atthe vertical center of the screen when the steering wheel is centered,the tracks generated by the two spots are inclined to the horizontalaxis. The caster angle for each wheel is equal to the angle between theline 210 or 212 traced by the spot reflected from that wheel on thescreen and the horizontal axis of the screen. Given a measurement of thecaster angle, corrections to the caster can be implemented as necessaryto secure compliance with the manufacturer's specifications.

In the event that the camber angles on the vehicle under test are notdesigned to be reset, the reflected beams must be centered vertically bymoving the vertical-axis leadscrews of the wheel-mounted retroreflectorkinematic mounts to secure a vertical retroreflector angle equal to theactual camber of the wheels. The positions to which the leadscrews mustbe set to center the reflected spots in the vertical plane then indicatethe actual camber angles of the left and right wheels.

In the event that it is necessary to align both the front and rearwheels of the vehicle being serviced, the mirrors 32 and 42 must bepartially transmitting, and lenses 45 should be installed as describedabove so as to equalize the displacements of the spots returned from thefront and rear retroreflectors due to errors in the orientation of thelongitudinal axis of the vehicle. Also, it will typically be necessaryto manually move mirrors 33 and 43 to a position opposite therear-mounted retroreflectors due to the variations in wheel base fromvehicle to vehicle. Mirrors 33 and 43 can easily be set up for use ifthe mirrors are set using levelled, indexed turning mounts on heavy-dutytripods of the appropriate height as described above in connection withFIGS. 7A-D.

Given these arrangements, the rear wheels of the vehicle can be alignedusing the same procedure as described above for the front wheels. In theevent that the toe-in and/or camber angles of the rear wheels are notdesigned to be reset, the existing toe-in and/or camber angles can bemeasured by observing the leadscrew settings at which the beamsreflected from the rear wheels are centered on the viewing screen.

In the case of vehicles with straight rear axles, the toe-in and camberangles of the rear wheels are identically zero. In this case, the beamsreflected from the rear wheels should merge when the vertical andhorizontal lead screws of the rear retroreflectors are set at zero.Note, however, that the merged spots may be offset in the horizontalplane from the center of the view screen due to the angular displacementof the longitudinal axis of the vehicle. Such displacement may be causedeither by errors in the placement of the vehicle or by damage to theframe or rear suspension system of the vehicle. If it is desired tocompensate for these effects, the alignment procedure for the frontwheels can be modified to require that the beams reflected from thefront wheels merge with the beams reflected from the rear wheels insteadof at the horizontal center of the viewing screen.

Conclusion

Each of the basic components used in the present invention, includingthe low power visible laser, dielectric or pellicle-type beamsplitters,refractive and fresnel lenses or holographic phase gratings, frontsurface mirrors and mirror, lens, and beamsplitter mounts are eithercommercially available or can be fabricated according to principleswhich are well known to workers in optics and precision mechanisms.

As noted in the description above, the central elements of the presentinvention are (1) the use of a laser source, beamsplitter, and set ofplane mirrors to create a system of laser reference beams incident onthe wheels being serviced, (2) a set of calibrated, tiltable,wheel-mounted retroreflectors set to null the angular offsets introducedby the specified toe-in and camber angles for each wheel, and (3) abeam-combining and imaging system permitting comparison of the angularorientation of the reflected beams on a single viewing screen.

Alignment is secured in this system when the combined beams merge toform a single, vertically centered spot on the viewing screen. Whileerrors in the placement of the vehicle on the slab will cause the mergedspots to move horizontally across the viewing screen, such errors willnot cause the merged spots to separate.

When assembled and used in the manner described above, this systemeliminates the requirement present in prior alignment systems for theuse of two or more independent gauges to measure the alignment of theindividual wheels during service. In addition, the system describedabove permits alignment service to be performed even when the axis ofthe vehicle has been offset due to an error in the positioning of thevehicle.

While the above is a complete description of the preferred embodiment ofthe present invention, various modifications, alternative constructions,and equivalents may be used. Therefore, the above description andillustrations should not be taken as limiting the scope of the presentinvention which is defined by the appended claims.

What is claimed is:
 1. Apparatus for use in setting desired toe-in andcamber angles on at least one pair of wheels on a vehicle, comprising:aplane mirror associated with each wheel; means for adjustably mountingear mirror to its associated wheel at a selected relative angularorientation that corresponds to the desired toe-in and camber angles forthat wheel such that when the wheels are set to the desired toe-in andcamber angles, the mirrors are vertical and parallel to each other andface outwardly; means for generating first and second beams; incidentbeam transport means for directing said beams toward respective outersurfaces of the wheels to be aligned so as to impinge on said mirrors,said beams being parallel to each other and in a reference plane whenthey impinge of said mirrors; and reflected beam transport means fordirecting the beams reflected from said mirrors to a common region, saidreflected beam transport means operating such that the reflected beamsoverlap at the common region when the toe-in and camber angles arecorrect.
 2. The apparatus of claim 1 wherein said means for generatingfirst and second beams comprises:a laser for generating a single beam;and a beam splitter disposed in the path of said single beam to providesaid first and second beams.
 3. The apparatus of claim 1 wherein saidincident beam transport means comprises first and second sets of planedeflection mirrors, disposed in the paths of said first and secondbeams, respectively.
 4. The apparatus of claim 3 wherein said reflectedbeam transport means comprises at least some of the same deflectionmirrors as are in said first and second sets of deflection mirrors. 5.The apparatus of claim 1 wherein said means for mounting comprises:areference plate; means for registering said reference plate to theassociated wheel in a plane substantially parallel to the plane of theassociated wheel; means for holding said reference plate in saidregistered position; and a kinematic mount for mounting the associatedmirror to said reference plate while allowing vertical and horizontalangular displacements therebetween.
 6. The apparatus of claim 5 whereinsaid registering means includes at least three pins extendingperpendicular to said reference plate.
 7. The apparatus of claim 5wherein said holding means comprises a plurality of permanent magnetsmounted to said reference plate at locations commensurate with thewheel's studs.
 8. The apparatus of claim 1, and further comprising:aviewing screen located at the common region.
 9. The apparatus of claim 1wherein said first and second beams are associated with opposite sidesof the vehicle, the vehicle has a front wheel and a rear wheel on eachside, and said incident beam transport means comprises, for each of saidfirst and second beams;first and second plane deflection mirrorsdisposed in the path of the associated beam and oriented to directrespective first and second portions of the associated beam toward thefront and rear wheels on the associated side for impingement alongparallel axes.
 10. The apparatus of claim 9 wherein said reflected beamtransport means comprises, for each of said first and second beams:thesame first and second deflection mirrors as are in said incident beamtransport means; and a lens, disposed in the path of said second beamportion only, said lens being at a location and having a focal length soas to cause the beams reflected from the front and rear wheels tooverlap at the common region, even if the vehicle is skewed relative tothe apparatus.
 11. Apparatus for use in setting desired toe-in andcamber angles on at least one pair of wheels on a vehicle, comprising:aplane mirror associated with each wheel; means for adjustably mountingeach mirror to its associated wheel at a selected relative angularorientation that corresponds to the desired toe-in and camber angles forthat wheel such that when the wheels are set to the desired toe-in andcamber angles, the mirrors are vertical and parallel to each other andface outwardly; and means for generating a set of laser reference beamsthat impinge on said mirrors, are reflected from said mirrors, and arebrought to a common region, said reference beams being parallel to eachother and in a reference plane when they impinge on said mirrors suchthat the reference beams overlap each other at the common region whenthe toe-in and camber angles of the wheels are correct.